Inkjet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand (DOD) inkjet, individual droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the ejection of ink droplets in drop-on-demand printing include thermal bubble formation (thermal inkjet (TIJ)) and piezoelectric transducers. In another process known as continuous inkjet (CIJ), a continuous stream of droplets is generated and expelled in an image-wise manner onto the surface of the image-recording element, while non-imaged droplets are deflected, caught and recycled to an ink sump. Inkjet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling.
Most types of inkjet printers employ a printing head made from silicon-based materials including silicon, silicon dioxide, and silicon nitride because these materials are common in semiconductor fabrication facilities and can be readily processed to form highly complex integrated circuits and electromechanical devices. Parts of the print head including the printing nozzles as well as the channels that feed ink to the print head and printing nozzles often contain regions of these silicon-based materials that are in direct contact with the printing ink. It is well known in the art that a wide range of ink compositions can cause these silicon-based materials to dissolve or induce stress that results in mechanical failure and increased rates of dissolution (U.S. Pat. No. 6,730,149 B2). The reliability of the inkjet printing device can be dramatically reduced by these interactions between the ink and the silicon-based materials in the print head.
This problem has been difficult to solve. In some inkjet printing systems, the silicon-containing portions of the print head, in particular the ink chamber, the nozzles, and the ink channels were replaced along with the ink cartridge so that the lifetime of the silicon-based materials was limited to the lifetime of the individual ink cartridge. This approach dramatically increases the cost of the ink cartridge and limits the printing system design.
Another approach to preventing the ink from dissolving the silicon-based materials has been to coat or deposit a resistant material on all the surfaces that come in contact with the ink. These coatings can be either organic such as polymers or inorganic such as oxides of titanium or hafnium. This method also has the drawback of increasing the cost of the print head and often is also plagued by poor uniformity or pinholes in the coating that limit the protection from the ink.
There is a strong need for ink compositions that do not dissolve or damage the silicon-based materials in the print head.
Ink compositions containing colorants used in inkjet printers can be classified as either pigment-based, in which the colorant exists as pigment particles suspended in the ink composition, or as dye-based, in which the colorant exists as a fully solvated dye species that consists of one or more dye molecules. Pigments are highly desirable since they are far more resistant to fading than dyes. However, pigment-based inks have a number of drawbacks. Great lengths must be undertaken to reduce a pigment to a sufficiently small particle size and to provide sufficient colloidal stability to the particles. Pigment-based inks often require a lengthy milling operation to produce particles in the sub-micron range needed for most modern ink applications. If the pigment particles are too large light scattering can have a detrimental effect on optical density and gloss in the printed image.
A second drawback of pigmented inks is their durability after printing, especially under conditions where abrasive forces have been applied to the printed image. Furthermore, the images printed onto an inkjet receiver are susceptible to defects at short time intervals, from immediately after printing to several minutes while the inks are drying. Finally, the durability of the dried image is also subject to environmental factors such as temperature and humidity which, under certain circumstances, can degrade image durability.
To this extent, pigmented inks have been formulated with various polymers, dispersants and other addenda in attempts to provide durable images that can withstand post printing physical abuse and environmental conditions.
A number of approaches to reducing the propensity of the ink to dissolve or damage the silicon-based print head materials have been disclosed. These methods typically involve controlling the alkali metal ion concentration, adding inhibiting organic molecules such as specific dyes, or adding cationic onium salts (U.S. Pat. No. 6,730,149 B2). Another approach employs dispersions of specific metal oxide particles such as alumina or cerium oxide where the particles have a positive charge in the pH range from 4 to 6 (US2008/0129811 A1) as characterized by their positive zeta potential in this pH range. These positively charged particles are believed to adhere to the negatively charge silicon-based surfaces in the print head and thereby eliminate the dissolution of these surfaces by the ink. These methods are limited to specific ink compositions and may not work well with pigment-based inks where a negative charge on the pigment surface and polymers is critical to the stability of the pigment dispersion and resulting ink. Another general approach to improving ink performance with regard to silicon corrosion is through adjustment of the ink pH value through the use of appropriate buffer solutions. For example, Inoue et al. in U.S. Pat. No. 7,370,952 B2 note that buffers can be used to adjust the pH values of inks used in drop-on-demand inkjet printers to reduce the effects of corrosion. This is primarily because the corrosion of silicon is known to be accelerated by higher pH value (more alkaline) solutions, such as those used in wet etching processes. At the same time, compositions useful to inkjet inks often require some alkalinity in order to maintain solution integrity, e.g., in order to prevent precipitation of ink components.
It has been found that low levels of some polyvalent metal ions such as aluminum can reduce the damage to the various materials in the print head (U.S. Pat. No. 6,607,268 B2)). If these ions can be effective at a low enough concentration so that they do not interact with other components of the ink, especially negatively charged pigments and polymers, these materials can be effective.